Various means have been used in the past to produce hidden or covert images. The most obvious utility for such covert images is perhaps the use of secret messages by agents of a political entity. Use of such covert images, however, is not necessarily primarily used for espionage, but are also used as an aid in prevention of fraudulent or counterfeit negotiable and/or non-negotiable documents and for determining winners in certain kinds of games and contests.
In the prior art, one known method for providing a covert image is to print a document with a background pantograph image, which contains a chemical ingredient which changes color when a bleach solution or other "ink" eradicator is applied. This means is not desirable as the application of a second chemical ingredient is necessary in order to cause the image to become visible. Thus, this means would not be suitable, for example, for games or contests, in which the general public desiring to reveal the covert image would not have access to such a chemical.
Another method known in the prior art is to print a document with an ink which contains a component which is colorless in visible light, but which fluoresces in a visible color when exposed to ultraviolet light. This method is also undesirable, in that it requires the party desiring to reveal the covert image to have access to additional equipment, i.e. a source of ultraviolet light.
A third manner known in the prior art to obtain a covert image is to print an image form on a document with an ink containing an abrasive pigment. When a coin or other metallic instrument is rubbed over the image, metal particles sare removed from the rubbing instrument and retained on the image, thus darkening the image. This method has the disadvantage that it is not possible to totally hide the image. The image which is intended to be covert can be seen to some extent, thus making it inappropriate for most end uses.
A fourth method of preparing covert images is to print an image on a substrate and then apply an opaque coating over it to hide it. The image is later made visible by scratching or rubbing away the overcoating to expose the image. This method is undesirable in view of the mess that is made by the scratched off coating.